This invention relates to a method of manufacturing cellulose beads and cellulose beads produced thereby.
Cellulose beads and cellulose derivative beads (hereinafter comprehensively referred to as cellulose beads) are used as ion exchangers, packing materials for chromatography, adsorbents for heavy metal ions and proteins, cosmetic additives, carriers for immobilization of biocatalyst, etc.
Commonly for these applications, such cellulose beads have to have high mechanical strength in conformity with their intended use, have to be as near to spheres as possible, and have to be as uniform in bead size as possible, i.e. as narrow in bead size distribution as possible.
Cellulose beads have to be as near to spheres as possible because the lower their sphericity, the larger the contact area between beads, and thus the smaller the exposed effective surface area of the beads. A large contact area between beads also leads to increased wear between beads and thus decrease in their mechanical strength. On the other hand, if beads that are rather wide in bead size distribution are put in a column or a tank as ion exchangers or packing material for chromatography, smaller beads may close gaps between larger beads, thus reducing the flow rate and increasing the pressure loss and thus the possibility of breakage of beads.
Also, it is a recent trend for customers to specify the bead size distribution as well as the average bead size. To meet this requirement, beads have to be separated e.g. on sieves. Such a separation step aggravates yield and manufacturing efficiency.
Conventional methods for manufacturing cellulose beads are roughly classified into two groups. Methods in the first group are chemical methods in which a cellulose solution is stirred in a dispersion medium to disperse small droplets of the cellulose solution utilizing phase separation between a cellulose solution and a dispersion medium, and coagulating the droplets to form cellulose beads. Methods in the second group are physical or mechanical methods in which a cellulose solution is sprayed with high pressure gas through nozzles and droplets thus formed are coagulated or dried to obtain cellulose beads.
Methods in the first group are disclosed in the following three patent publications. Firstly examined Japanese patent publication 57-45254 discloses a method of manufacturing spherical cellulose beads. This method comprises the steps of continuously stirring a suspension of viscose in a water immiscible solvent having a viscosity of 100 centistokes (cst) or less such as chlorobenzene while heating it to 30-100.degree. C. until the suspension solidifies into beads, and subjecting the beads to acid-hydrolysis to produce spherical cellulose beads.
In the method disclosed in examined Japanese patent publication 55-39565, a methylene chloride or chloroform solution of cellulose triacetate is added dropwise while stirring into an aqueous medium obtained by dissolving in water a stabilizer such as a gelatine or a polyvinyl alcohol, and the mixture is heated to obtain spherical beads of cellulose triacetate, and the beads obtained are saponified to produce intended cellulose beads.
Examined Japanese patent publication 6-62781 discloses a method of producing porous cellulose beads comprising the steps of blending an alkaline aqueous polymer solution of cellulose xanthate and a water soluble polymer other than cellulose xanthate with an anionic polymer to produce a suspension of an alkaline aqueous polymer solution, and coagulating the suspension by heating or adding a cellulose xanthate coagulant, and then neutralizing it with an acid to regenerate cellulose beads while simultaneously removing the water soluble polymer from the cellulose beads, or coagulating and neutralizing the suspension with an acid to regenerate cellulose beads while simultaneously removing the water soluble polymer from the cellulose beads.
A method in the second group is disclosed in examined Japanese patent publication 56-21761. In this method, a viscose or cellulose cuprammonium solution is continuously extruded through a discharge nozzle into the atmosphere so that the solution is spontaneously split, and the split droplets of the cellulose solution are fed into a coagulating/regenerating bath.
Another method in the second category is disclosed in unexamined Japanese patent publication 4-41533. In this method, a solution of sodium cellulose xanthate is sprayed into hot blast with an atomizer such as a twinfluid atomizer or a rotary nozzle and dried to produce cellulose beads.
The methods in the first category require precise preparation of the cellulose solution, and strict control of the stirring speed. These methods are thus difficult to carry out. Also, since these methods are all batch type, production efficiency is low.
The methods in the second category need no complicated equipment. Beads can be produced continuously. But these methods have other problems. One problem is that a liquid sprayed in mist into an air stream forms numerous beads that are irregular in shape and size, thus widening the bead size distribution. Also, since the cellulose solution is sprayed under high pressure in the form of minute droplets, the droplets tend to be scattered at high speed until they come into contact with a coagulating solution. This increases the possibility of secondary fragmentation of beads, so that air bubbles tend to mix into the beads to form relatively large pores therein. This lowers the mechanical strength of the beads.
An object of this invention is to provide a method of producing cellulose beads and porous cellulose beads which are free of these problems of the conventional methods, and which can produce cellulose beads that are high in sphericity and narrow in bead size distribution, and to provide such cellulose beads by the method.